Structure and method to fabricate high performance MTJ devices for spin-transfer torque (STT)-RAM

ABSTRACT

A STT-RAM MTJ is disclosed with a MgO tunnel barrier formed by a NOX process, a CoFeB/FeSiO/CoFeB composite free layer with a middle nanocurrent channel layer to minimize Jc 0 , and a Ru capping layer to enhance the spin scattering effect and increase dR/R. Good write margin is achieved by modifying the NOX process to afford a RA less than 10 ohm-μm 2  and good read margin is realized with a dR/R of &gt;100% by annealing at 330° C. or higher to form crystalline CoFeB free layers. The NCC thickness is maintained in the 6 to 10 Angstrom range to reduce Rp and avoid Fe(Si) granules from not having sufficient diameter to bridge the distance between upper and lower CoFeB layers. A FeSiO layer may be inserted below the Ru layer in the capping layer to prevent the Ru from causing a high damping constant in the upper CoFeB free layer.

RELATED PATENT APPLICATIONS

This application is related to the following: Ser. No. 12/079,445,filing date Mar. 27, 2008; and Ser. No. 12/082,155, filing date Apr. 9,2008; both of which are assigned to a common assignee and are hereinincorporated by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to a high performance Magnetic Tunneling Junction(MTJ) element and a method for making the same, and in particular, to aspin transfer (Spin-RAM) device that achieves low switching current andhigh dR/R by incorporating a free layer having a FeCoB/FeSiO/FeCoBconfiguration in which the thinner, upper FeCoB layer is easier toswitch than the lower, thicker FeCoB layer and the single MTJ functionslike a dual spin filter (DSF).

BACKGROUND OF THE INVENTION

Magnetoresistive Random Access Memory (MRAM), based on the integrationof silicon CMOS with MTJ technology, is a major emerging technology thatis highly competitive with existing semiconductor memories such as SRAM,DRAM, and Flash. Similarly, spin-transfer (spin torque) magnetizationswitching described by C. Slonczewski in “Current driven excitation ofmagnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), hasrecently stimulated considerable interest due to its potentialapplication for spintronic devices such as STT-RAM on a gigabit scale.

As shown in FIG. 1, one embodiment of a memory cell in a STT-RAM 1includes a gate 5 formed above a p-type semiconductor substrate 2, asource 3, drain 4, word line (WL) 7 above the gate, and a source line 9.There is also a bottom electrode (BE) 10 formed above the source line 9and word line 7, and a MTJ cell 11 between the BE and bit line (BL) 12.There is a Cu stud 6 connecting the source 3 to BL 12, and a via 13 andCu stud 8 to connect BE 10 to drain 4. Thus, the transistor source 3 anddrain 4 are connected to the MTJ 11 so that DC current may flow acrossthe MTJ.

Both MRAM and STT-RAM have a MTJ element based on a tunnelingmagneto-resistance (TMR) effect wherein a stack of layers has aconfiguration in which two ferromagnetic layers are separated by a thinnon-magnetic dielectric layer. The MTJ element is typically formedbetween a bottom electrode such as a first conductive line and a topelectrode which is a second conductive line at locations where the topelectrode crosses over the bottom electrode. A MTJ stack of layers mayhave a bottom spin valve configuration in which a seed layer, ananti-ferromagnetic (AFM) pinning layer, a ferromagnetic “pinned” layer,a thin tunnel barrier layer, a ferromagnetic “free” layer, and a cappinglayer are sequentially formed on a bottom electrode. The AFM layer holdsthe magnetic moment of the pinned layer in a fixed direction. The pinnedlayer has a magnetic moment that is fixed in the “x” direction, forexample, by exchange coupling with the adjacent AFM layer that is alsomagnetized in the “x” direction. The free layer has a magnetic momentthat is either parallel or anti-parallel to the magnetic moment in thepinned layer. The tunnel barrier layer is thin enough that a currentthrough it can be established by quantum mechanical tunneling ofconduction electrons. The magnetic moment of the free layer may changein response to external magnetic fields and it is the relativeorientation of the magnetic moments between the free and pinned layersthat determines the tunneling current and therefore the resistance ofthe tunneling junction. When a sense current is passed from the topelectrode to the bottom electrode in a direction perpendicular to theMTJ layers, a lower resistance is detected when the magnetizationdirections of the free and pinned layers are in a parallel state (“1”memory state) and a higher resistance is noted when they are in ananti-parallel state or “0” memory state.

In a read operation, the information stored in a MRAM cell is read bysensing the magnetic state (resistance level) of the MTJ element througha sense current flowing top to bottom through the cell in a currentperpendicular to plane (CPP) configuration. During a write operation,information is written to the MRAM cell by changing the magnetic statein the free layer to an appropriate one by generating external magneticfields as a result of applying bit line and word line currents in twocrossing conductive lines, either above or below the MTJ element. Oneline (bit line) provides the field parallel to the easy axis of the bitwhile another line (digit line) provides the perpendicular (hard axis)component of the field. The intersection of the lines generates a peakfield that is engineered to be just over the switching threshold of theMTJ.

A high performance MRAM MTJ element is characterized by a high tunnelingmagnetoresistive (TMR) ratio which is dR/R where R is the minimumresistance of the MTJ element and dR is the change in resistanceobserved by changing the magnetic state of the free layer. A high TMRratio and resistance uniformity (Rp_cov), and a low switching field (Hc)and low magnetostriction (λ_(S)) value are desirable for conventionalMRAM applications. For Spin-RAM (STT-RAM), a high λ_(S) and high Hcleads to high anisotropy for greater thermal stability. This result isaccomplished by (a) well controlled magnetization and switching of thefree layer, (b) well controlled magnetization of a pinned layer that hasa large exchange field and high thermal stability and, (c) integrity ofthe tunnel barrier layer. In order to achieve good barrier propertiessuch as a specific junction resistance x area (RA) value and a highbreakdown voltage (Vb), it is necessary to have a uniform tunnel barrierlayer which is free of pinholes that is promoted by a smooth and denselypacked growth in the AFM and pinned layers. RA should be relativelysmall (<10000 ohm-μm²) for MTJs that have an area defined by an easyaxis and hard axis dimensions of less than 1 micron. Otherwise, R wouldbe too high to match the resistance of the transistor which is connectedto the MTJ.

As the size of MRAM cells decreases, the use of external magnetic fieldsgenerated by current carrying lines to switch the magnetic momentdirection becomes problematic. One of the keys to manufacturability ofultra-high density MRAMs is to provide a robust magnetic switchingmargin by eliminating the half-select disturb issue. For this reason, anew type of device called a spin transfer (spin torque) device wasdeveloped. Compared with conventional MRAM, spin-transfer torque orSTT-RAM has an advantage in avoiding the half select problem and writingdisturbance between adjacent cells. The spin-transfer effect arises fromthe spin dependent electron transport properties offerromagnetic-spacer-ferromagnetic multilayers. When a spin-polarizedcurrent transverses a magnetic multilayer in a CPP configuration, thespin angular moment of electrons incident on a ferromagnetic layerinteracts with magnetic moments of the ferromagnetic layer near theinterface between the ferromagnetic and non-magnetic spacer. Throughthis interaction, the electrons transfer a portion of their angularmomentum to the ferromagnetic layer. As a result, spin-polarized currentcan switch the magnetization direction of the ferromagnetic layer if thecurrent density is sufficiently high, and if the dimensions of themultilayer are small. The difference between a STT-RAM and aconventional MRAM is only in the write operation mechanism. The readmechanism is the same.

For STT-RAM to be viable in the 90 nm technology node and beyond, theultra-small MTJs (also referred to as nanopillars or nanomagnets herein)must exhibit a TMR ratio that is much higher than in a conventionalMRAM-MTJ which uses AlOx as the tunnel barrier and a NiFe free layer.Furthermore, the critical current density (Jc) must be lower than about10⁶ A/cm² to be driven by a CMOS transistor that can typically deliver100 μA per 100 nm gate width. A critical current for spin transferswitching (Ic), which is defined as [(Ic⁺+Ic⁻I)/2], for the present 180nm node sub-micron MTJ having a top-down area of about 0.2×0.4 micron,is generally a few milliamperes. The critical current density (Jc), forexample (Ic/A), is on the order of several 10⁷ A/cm². This high currentdensity, which is required to induce the spin-transfer effect, coulddestroy a thin tunnel barrier made of AlOx, MgOx, or the like. Thus, forhigh density devices such as STT-RAM on a gigabit scale, it is desirableto decrease Ic (and its Jc) by approximately an order of magnitude so asto avoid an electrical breakdown of the MTJ device and to be compatiblewith the underlying CMOS transistor that is used to provide switchingcurrent and to select a memory cell.

A. Fert et al. in “Magnetization reversal by injection and transfer ofspin: experiments and theory”, J. Magn. Magn. Materials, Vol. 272, p.1706 (2004), point out that a larger reduction in switching currentdensity may be realized with magnetic materials permitting high spinaccumulations. Another type of spin transfer effect is described as acurrent-induced domain wall motion. Further, O. Ozatay et al. in “Spintransfer by nonuniform current injection into a nanomagnet”, Appl. Phys.Lett., 88, 202502 (2006), state that current injected throughnanochannels results in high current density which causes reverse domainnucleation. The domain wall will be pushed out by the continuousinjection current until the entire free layer switches.

Once a certain MTJ cell has been written to, the circuits must be ableto detect whether the MTJ is in a high or low resistance state which iscalled the “read” process. Uniformity of the TMR ratio and the absoluteresistance of the MTJ cell are critical in MRAM (and STT-RAM)architecture since the absolute value of MTJ resistance is compared witha reference cell in a fixed resistance state during read mode. Needlessto say, the read process introduces some statistical difficultiesassociated with the variation of resistances of MTJ cells within anarray. If the active device resistances in a block of memory show alarge resistance variation (i.e. high Rp_cov, Rap_cov), a signal errorcan occur when they are compared with a reference cell. In order to havea good read operation margin, TMR/Rp_cov (or Rap_cov) should have aminima of 12, preferably >15, and most preferably >20 where Rp is theMTJ resistance for free layer magnetization aligned parallel to pinnedlayer magnetization (which is fixed) and Rap is the resistance of freelayer magnetization aligned anti-parallel to the pinned layermagnetization.

The intrinsic critical current density (Jc) as given by Slonczewski ofIBM is shown in equation (1) below.Jc=2eαMst _(F)(Ha+H _(k)+2πMs)/

η  (1)where e is the electron charge, α is a Gilbert damping constant, t_(F)is the thickness of the free layer,

is the reduced Plank's constant, η is the spin-transfer efficiency whichis related to the spin polarization (P), Ha is the external appliedfield, H_(k) is the uniaxial anisotropy field, and 2πMs is thedemagnetization field of the free layer. Two publications by C.Slonczewski that relate to STT-RAM are entitled “Current drivenexcitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7(1996), and “Current, torques, and polarization factors in magnetictunnel junctions”, Physical Review B 71, 024411 (2005). In a MTJstructure (F/I/F) where F is a ferromagnetic layer and I is aninsulator, when the spin relaxation distance is much larger than theferromagnetic film thickness, the spin continuity holds true, i.e., thesum of interfacial torques from both left and right sides equals the netinflow of spin current. As the magnetization is fixed on one side, theother side magnetization will experience an in-plane torque of T=−(

P_(L)J₀/2e)sin(θ) where e is the electron charge, P_(L) is tunnelingpolarization parameter, J₀ is electric current density, and θ is theangle between the magnetizations on the two sides of the tunnel barrier(insulator).

Normally, the demagnetizing field, 2πMs (several thousand Oe term) ismuch larger than the uniaxial anisotropy field Hk and external appliedfield (approximately 100 Oe) Ha term, hence the effect of Hk and Ha onJc are small. In equation (2), V equals Ms(t_(F)A) and is the magneticvolume which is related to the thermal stability function termK_(u)V/k_(b)T where K_(u) is the magnetic anisotropy energy and k_(b) isthe Boltzmann constant.Jc∝αMsV/

η  (2)

Another publication relating to a STT-RAM (Spin-RAM) structure is by M.Hosomi et al. in “A novel non-volatile memory with spin torque transfermagnetization switching: Spin-RAM”, 2005 IEDM, paper 19-1, and describesa 4 Kbit Spin RAM having CoFeB pinned and free layers, and aRF-sputtered MgO tunnel barrier that was annealed under 350° C. and10000 Oe conditions. The MTJ size is 100 nm×150 nm in an oval shape. Thetunnel barrier is made of crystallized (001) MgO with a thicknesscontrolled to <10 Angstroms for a proper RA of around 20 ohm-um².Intrinsic dR/R of the MTJ stack is 160% although dR/R for the 100 nm×150nm bit during read operation (with 0.1 V bias) is about 90% to 100%.Using a 10 ns pulse width, the critical current density, Jc, for spintransfer magnetization switching is around 2.5×10⁶ A/cm². Write voltagedistribution on a 4 Kbit circuit for high resistance state to lowresistance (P to AP) and low resistance state to high resistance state(AP to P) has shown good write margin. Resistance distribution for thelow resistance state (Rp) and high resistance state (Rap) has a sigma(Rp_cov) of about 4%. Thus, for a read operation, TMR (with 0.1 Vbias)/Rp_cov is >20.

H. Meng and J. Wang in “Composite free layer for high density magneticrandom access memory with low spin transfer current”, APL Vol. 89, pp.152509 (2006), fabricated two sets of MTJs. In a first MTJ representedby Si/SiO₂/BE/Ta/IrMn/CoFe/AlOx/CoFe30/Ta/top electrode, they employ asingle CoFe free layer that is 30 Angstroms thick. In a second MTJrepresented bySi/SiO₂/BE/Ta/CoFe20/FeSiO30/CoFe10/AlOx/CoFe/Ru/CoFe/IrMn/Ta/topelectrode, there is a composite free layer with a nanocurrent channel(NCC) FeSiO layer sandwiched between two CoFe layers. RA values are 4.2ohm-μm² and 7 ohm-μm², and TMR ratios are around 16.5% and 10% for thefirst MTJ and second MTJ, respectively. It is interesting to note thatthe Jc₀ value (by extrapolation) of 8×10⁶ A/cm² for the second MTJ isabout 33% that of the first MTJ (2.4×10⁷ A/cm²).

In two related publications by Y. Jiang et al., entitled “Perpendiculargiant magnetoresistance and magnetic switching properties of a singlespin valve with a synthetic antiferromagnet as a free layer”, Phys. Rev.B, Vol. 68, p. 224426 (2003), and “Effective reduction of criticalcurrent for current-induced magnetization switching by a Ru layerinsertion in an exchange-biased spin valve”, PRL, V. 92, p. 167204(2004), that relate to current induced magnetization switching (CIMS) innanopillar CPP-GMR structures, a thin Ru layer formed on a CoFe freelayer was found to considerably lower the critical current (Jc).

To reduce the resistance RA to ˜10 ohm-μm² and minimize the chance ofelectrical breakdown, the thickness of MgO formed by RF magnetronsputtering of a MgO target was set at 8.5 Angstroms by J. Hayakawa etal. in “Current-driven magnetization switching in CoFeB/MgO/CoFeBmagnetic tunnel junction”, Japn. J. Appl. Phys., V 44, p. 1265 (2005).They noted that a CoFeB free layer is amorphous after 270° C. or 300° C.annealing but is crystalline following 350° C. annealing. Criticalcurrent density Jc with a 10 ns pulse width required for current drivenswitching was as low as 7.8×10⁵ A/cm² for the 270° C. annealed sample.

T. Kawahara et al. in “2 Mb Spin-transfer Torque RAM with bit-by-bitdirectional current write and parallelizing-direction current read”,2007 IEEE International Solid State Circuits Conference, describe aSTT-RAM having a CoFeB/RF sputtered MgO/CoFe—NiFe pinned/barrier/freelayer configuration. MgO thickness is 10 Angstroms to give a RA of 20ohm-μm². Annealing temperature is 350° C. and TMR is about 100%.Switching voltage for the 100 nm×50 nm oval MTJ using a 100 ns pulse isaround 0.7 V.

The references above clearly indicate that spin-transfer-torque writingis a viable candidate for low power, high density non-volatile RAM. TheMTJ structure for STT-RAM typically uses a CoFeB pinned layer, a RFsputter deposited MgO layer, and a CoFeB or CoFe/NiFe free layer. Notethat nano-scale junctions of all the referenced MTJs were fabricatedusing electron beam lithography and ion beam etching (IBE) processes.However, the nanopillars formed by IBE results in MTJ elements with asloping profile are not suitable for making very high density (Gbit)STT-RAMs. Further improvements are needed, and in particular, ananoscaled MTJ that is made by optical lithography and reactive ion etch(RIE) processes to allow more vertical MTJ profiles, a high TMR ratio, aRA value of less than 15 ohm-μm², and a low Jc is desirable.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide a MTJ for anSTT-RAM that delivers a high dR/R of about 100% with a RA of less thanabout 15 ohm-μm², and achieves a low Jc₀ of about 2.5×10⁶ A/cm² or lessto facilitate magnetization switching of the free layer withoutnegatively affecting the tunnel barrier layer.

A second objective of the present invention is to provide a fabricationmethod for forming a high density STT-RAM comprised of a MTJ accordingto the first objective.

According to one embodiment, these objectives are achieved by providinga substrate comprised of a bottom conductor electrode (BE) on which aspin-transfer (STT)-RAM structure is to be fabricated. The BE ispreferably comprised of a TaN/NiCr/Ru/Ta configuration where the Talayer is sputter etched to form a amorphous surface which promotessmooth and flat overlying layers in the MTJ. Once the BE is patterned toform an array of lines, a thin oxygen surfactant layer (OSL) is formedon the BE. Thereafter, a MTJ stack of layers is deposited on the BE/OSLstack. In one aspect, the MTJ stack has a bottom spin valveconfiguration in which a seed layer, AFM layer, pinned layer, tunnelbarrier layer, free layer, and a capping layer/hard mask layer aresequentially formed on the BE/OSL. The seed layer is preferably NiCr andthe AFM layer may be comprised of MnPt or IrMn, for example. Preferably,the pinned layer has a CoFe/Ru/CoFeB/CoFe or CoFe/Ru/CoFeB configurationwhere the former is used with an amorphous CoFeB free layer and thelatter is employed with a crystalline CoFeB free layer.

The tunnel barrier layer is preferably comprised of crystalline MgO madeby a natural oxidation method in order to minimize the RA value. A keyfeature is that the free layer is a composite comprised of a nanocurrentchannel (NCC) layer such as FeSiO sandwiched between two Co₄₀Fe₄₀B₂₀layers wherein the lower CoFeB layer is thicker than the upper layer.Preferably, there is a thin Ru capping layer formed on the free layersuch that the Ru serves as a spin scattering layer. A Ta hard mask maybe formed on the Ru capping layer to complete the MTJ stack of layers.In a second embodiment, the capping layer has a FeSiO/Ru configurationwhich is used to lower the damping constant of the upper CoFeB layer inthe composite free layer.

All of the layers in the MTJ stack may be formed by sputtering or ionbeam deposition (IBD). Once all the layers in the MTJ stack are laiddown, a thermal anneal process may be employed to fix the pinned layermagnetization (easy-axis) direction. The MTJ stack is patterned with aphotolithography step to define a MTJ shape in a photoresist maskinglayer followed by a RIE process to transfer the MTJ shape through theMTJ stack and form a MTJ nanopillar having essentially verticalsidewalls that enable highly dense MTJ arrays necessary for Gbit STT-RAMdevices.

In one embodiment, a photoresist layer is formed on the hard mask andpatterned to define an array of nanomagnet (MTJ) shapes from a top-view.Then a reactive ion etch (RIE) process is employed to etch throughportions of the hard mask that are not covered by the photoresist layer.Thereafter, the first photoresist layer may be removed and a second RIEstep is used to selectively etch through portions of the MTJ stack thatare not protected by the hard mask. To complete the STT-RAM structure, asecond dielectric layer such as silicon oxide is deposited on the MTJelement and surrounding substrate. A chemical mechanical polish (CMP)process is used to remove a top portion of the second dielectric layerthereby exposing the hard mask and making the second dielectric layercoplanar with the top of the MTJ element. A bit line array is thenformed on the second dielectric layer by depositing a conductive layerfollowed by employing a photolithography patterning and RIE sequence todelineate a bit line on the hard mask.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a memory cell in a conventionalSTT-RAM device.

FIG. 2 is a cross-sectional view of a STT-RAM with a MTJ nanopillarformed according to an embodiment of the present invention.

FIG. 3 is an enlarged cross-sectional view of the composite free layerin the MTJ nanopillar from FIG. 2 and depicts a nano-current channellayer formed therein.

FIG. 4 is an enlarged cross-sectional view of a portion of a compositecapping layer in a MTJ nanopillar according to one embodiment of thepresent invention.

FIG. 5 is an enlarged cross-sectional view of a portion of a compositehard mask in a MTJ nanopillar according to an embodiment of the presentinvention.

FIG. 6 is a drawing showing how devices with low R (high lap) in graph(a) relate to data points on graph (b) where switching voltage isplotted vs. breakdown voltage.

FIGS. 7 a, 7 b are graphs showing an Rp distribution in nanomagnetscomprised of a CoFeB/FeSiO/CoFeB free layer.

FIG. 8 is a cross-sectional view of a STT-RAM with a MTJ nanopillarformed according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a MTJ element (nanopillar) for an STT-RAMapplication that provides a combination of high dR/R, low RA, and lowcritical current density which is necessary for enhanced performance inhigh density STT-RAMs. The present invention also provides a fabricationsequence for a STT-RAM on an ultra high density scale. Drawings areprovided by way of example and are not intended to limit the scope ofthe invention. Although the exemplary embodiment depicts a bottom spinvalve configuration, the present invention also encompasses a top spinvalve design as appreciated by those skilled in the art. A “top view” asdescribed herein is defined as a viewpoint from a position above theplane of the substrate in the STT-RAM device.

Referring to FIG. 2, one embodiment of a MTJ according to the presentinvention is depicted. In particular, a MTJ comprised of layers 33-42 isformed between a bottom electrode 32 and a bit line 43 in an MRAM array(not shown). The bottom electrode (BE) 32 may be a composite layercomprised of a lower seed layer, middle conductive layer, and uppercapping layer. Preferably, the BE 32 has a TaN/NiCr/Ru/Ta configurationin which the lower TaN layer (not shown) is formed on a substrate thatmay be an insulation layer 31 comprised of silicon oxide or the like.The insulation layer 31 may have a metal stud 30 formed therein so thatelectrical contact can be established between bit line 43 and a wordline (not shown) in a sub-structure below the substrate. In a preferredBE configuration, the thicknesses of the TaN, NiCr, Ru, and Ta layersare 30, 30, 100, and 120 Angstroms, respectively. However, the thicknessof the various BE layers may be adjusted higher or lower to optimizeperformance as appreciated by those skilled in the art. The Ta layer(not shown) may be further subjected to sputter etching or ion millingto form an amorphous surface. Subsequently, an oxygen surfactant layer(OSL) may be formed on the α-Ta surface according to a method previouslydescribed in U.S. Pat. No. 7,208,807 which is herein incorporated byreference in its entirety.

An amorphous Ta top surface on BE 32 is especially advantageous inpromoting a uniform and dense growth in subsequently formed MTJ layers.The most critical layers in the MTJ stack are the tunnel barrier 39 andfree layer 40. The tunnel barrier 39 must be extremely uniform over thewafer since small variations in thickness will result in a largevariation in the resistance and in the RA value. In one embodiment, theBE 32 is patterned to form an array of BE lines before depositing an OSL(not shown) on the BE. Then, an insulation layer (not shown) isdeposited and planarized by a conventional method to become coplanarwith BE 32 (or BE/OSL stack).

In the exemplary embodiment, the MTJ stack is fabricated on thepatterned BE 32 by sequentially forming a seed layer 33, AFM layer 34,synthetic anti-ferromagnetic (SyAF) pinned layer 35, MgO tunnel barrier39, free layer 40, capping layer 41, and hard mask layer 42. Seed layer33 is preferably NiCr but may be comprised of NiFe, NiFeCr, or othersuitable seed layer materials and has a thickness between 40 to 60Angstroms. When a NiCr seed layer is grown on an oxygen surfactanttreated α-Ta surface in the BE 32, a smooth and dense (111) NiCr crystalorientation results which promotes smooth and densely packed growth insubsequently formed MTJ layers.

The AFM layer 34 is preferably comprised of MnPt with a thickness in therange of 120 to 200 Angstroms although an IrMn layer about 50 to 100Angstroms thick or other materials such as NiMn, OsMn, RuMn, RhMn, PdMn,RuRhMn, or MnPtPd are also acceptable. SyAF pinned layer 35 may have aAP2/coupling/AP1 configuration to improve thermal stability of the MTJand also reduce the interlayer coupling Hin (offset) field applied tothe free layer. Preferably, the AP2 layer 36 is made of CoFe, thecoupling layer 37 is Ru, and the AP1 layer 38 is comprised of CoFeB/CoFeor CoFeB. In one embodiment, a CoFeB/CoFe AP1 layer 38 is used incombination with an amorphous CoFeB free layer 40. In anotherembodiment, a CoFeB AP1 layer 38 is employed in combination with acrystalline CoFeB free layer 40. As described in a later section, acrystalline CoFeB free layer preferably has a CoFeB/NCC/CoFeBconfiguration wherein the NCC (nano-conducting channel) layer may besubstantially amorphous in character.

A critical feature of the present invention is the process of formingthe MgO tunnel barrier 39. Unlike a method commonly used in the priorart references where a MgO tunnel barrier is deposited by RF magnetronsputtering directly from a sintered MgO target, we advantageously employa procedure where a Mg layer about 6 to 8 Angstroms thick isDC-magnetron sputtered followed by an in-situ natural oxidation (NOX),and then sputter deposition of a second Mg layer about 3 to 5 Angstromsthick. Our method involving a DC sputtering process with a metallic Mgtarget results in a uniform Mg film that is particulate free. Thedesired RA value for the STT-RAM MTJ of less than about 15 ohm-μm² canbe achieved by adjusting Mg thickness and NOX process conditions.Typically, the NOX process is performed in an oxidation chamber within aDC-magnetron sputter deposition tool and comprises an oxygen flow rateof 0.1 to 1 standard liters per minute (slm) for a period of 100 to 600seconds. The oxygen pressure is about 1 torr. It is believed that theoxygen in the MgO layer resulting from the NOX process diffuses into thesecond Mg layer to form a uniform MgO layer in which essentially all ofthe first and second Mg layers are oxidized.

It is known by those skilled in the art that a MTJ made from acrystalline (001) MgO tunnel barrier and a CoFeB free layer is capableof delivering a very high dR/R that results from coherent tunneling inwhich electron symmetry of the ferromagnetic electrode is preserved intunneling through the crystalline (001) MgO tunnel barrier. The mostcommon CoFeB composition is represented by [Co_(X)Fe_((1-X))]₈₀B₂₀ wherethe content of B is 20 atomic % and x is the atomic % of Co. Theas-deposited CoFeB film has an amorphous phase structure and remainsamorphous unless recrystallization occurs by annealing above 300° C. Itshould be understood that even though a CoFeB free layer has a somewhathigher intrinsic damping constant than a comparable CoFe free layer, acrystalline (100) CoFeB free layer resulting from annealing above 300°C. has a high polarization that leads to significantly higher dR/R thana CoFe free layer. Since a Co₄₀Fe₄₀B₂₀ alloy has a higher Fe contentthan a Co₆₀Fe₂₀B₂₀ alloy, the former has a higher polarization but lowerdamping factor than the Co₆₀Fe₂₀B₂₀ alloy. According to equation (2)presented previously, a free layer having higher polarization and lowerdamping constant results in lower switching current density Jc. Thus, afree layer 40 comprised of Co₄₀Fe₄₀B₂₀ alloy is preferred in the presentinvention.

In related patent application Ser. Nos. 12/079,445 and 12/082,155, wedescribed a composite free layer having a CoFeB/FeSiO/CoFeBconfiguration in order to reduce Jc₀. In one aspect, a dual spin filter(DSF) structure may be employed to reduce Jc₀ but during fabrication ofthe DSF nanomagnets we observed considerable shorting in devices causedby patterning the nanomagnets by RIE. Although Jc₀ for a DSF nanomagnetmay be lowered to about 2.5×10⁶ A/cm², the dR/R was unfortunatelyreduced to 35% which is too low for STT-RAM. Thus, we were motivated toseek novel nanomagnets based on single spin valves where Jc₀ could belowered to about 2.5×10⁶ A/cm² while maintaining dR/R near 100%.

Referring to FIG. 3, free layer 40 is preferably a composite thatincludes a lower magnetic layer 50, a middle nanocurrent channel (NCC)layer 51 with a thickness t of 6 to 10 Angstroms, and an upper magneticlayer 52. The magnetic layers 50, 52 are preferably made of CoFeB havinga low magnetic damping constant. Upper magnetic layer 52 has a smallerthickness of about 6 to 8 Angstroms compared with the lower magneticlayer 50 which has a thickness between 10 and 15 Angstroms. Therefore,the upper magnetic layer 52 is easier to switch than the lower magneticlayer 50.

Another important feature of the present invention is the middle NCClayer 51 made of RSiO or RSiN where R is Fe, Co, Ni, B, or an alloythereof such as CoFeSiO, or a metal, and RSiO and RSiN are composites inwhich conductive R(Si) grains such as Fe(Si) are magnetically andelectrically coupled with the adjacent magnetic layers 50, 52, and areformed in an amorphous silicon oxide (or silicon nitride) insulatormatrix. The R(Si) grains such as Fe(Si) are typically formed in columnarshapes that may extend from the lower magnetic layer 50 to the uppermagnetic layer 52. The two magnetic layers 50, 52 are ferromagneticallycoupled and therefore have a magnetic moment in the same direction. Forinstance, the magnetization direction of magnetic layers 50, 52 mayeither be aligned parallel or anti-parallel to the magnetizationdirection of AP1 reference layer 38.

In the exemplary embodiment that has a CoFeB/FeSiO/CoFeB free layer 40configuration and a Ru capping layer 41, the spin current in the FeSiOportion of the free layer passes only through the nano-conductingchannels 51 a within a silicon oxide matrix 51 b. As a result, there ishigh current density about 9-fold greater than in typical free layerscomprised of CoFeB which will cause reverse domain nucleation asdescribed in the aforementioned Ozatay reference in which the domainwall will be pushed out by the continuous injection current until theentire free layer switches. Thermal heating caused by local currentdensity may also contribute to magnetization switching in the two CoFeBlayers. Moreover, the spin transfer mechanism described in theaforementioned Fert reference also contributes to magnetization reversalin an MTJ according to the present invention. Because of the existenceof reverse magnetization grains in the NCC layer and their coupling tothe CoFeB layers, the magnetization switching of the magnetic layers 50,52 will be much easier than when a NCC layer is not present in the MTJ.In effect, the single spin valve (MTJ) described herein behaves like aDSF (but without a reduction in dR/R) in that the spin polarized currentis reflected back from a free layer/cap layer interface and accumulatesin the free layer. The enhancement of the transverse spin accumulationincreases the spin torque and therefore effectively reduces theswitching current.

In an embodiment where the NCC layer 51 is FeSiO, deposition istypically accomplished by RF-magnetron sputtering from a Fe(25 atomic%)-SiO₂ single target to provide an NCC thickness from 6 to 15Angstroms, and preferably 6 to 10 Angstroms. According to S. Honda etal. in “Tunneling giant magnetoresistance in heterogeneous Fe—SiO₂granular films”, Phys. Rev. B. V 56, p 14566 (1997), the volume fraction(x) of the Fe(Si) granules in the SiO₂ matrix is calculated to be 0.115.The isolated Fe(Si) granules were found to have a diameter ranging from10 to 20 Angstroms. We have set the lower limit of granule diameterdistribution (10 Angstroms) as our process of record (POR)NCC layer 51thickness in order to ensure uniformity in the NCC channels 51 a betweenthe two magnetic layers 50, 52. When the NCC thickness is greater thanabout 10 Angstroms, then some of the Fe(Si) granules fail to function asnano-current channels which leads to a high resistance in NCC layer 51thereby increasing the RA of the MTJ with the composite free layer 40.High resolution transmission electron microscopy (HR-TEM) indicates a 10Angstrom thick FeSiO layer 51 is grown as a continuous film that is flatand smooth similar to MgO tunnel barrier layer 39.

Above the free layer 40 is a cap layer 41 comprised of Ru having athickness of 10 to 30 Angstroms, and preferably 30 Angstroms. A thin Rucapping layer 41 not only enhances dR/R but also considerably lowers thecritical current Jc₀. A substantial reduction in Jc is believed toresult primarily because the Ru layer serves as a strong spin scattererfor the majority of electrons which leads to an enhanced spinaccumulation at the CoFeB layer 52/capping layer 41 interface. Theenhanced spin accumulation will increase the polarized current insidethe free layer and thus produce an additional spin torque to act on theCoFeB magnetization.

In an alternative embodiment illustrated in FIG. 4, the cap layer 41 isa composite comprised of a lower FeSiO layer 41 a about 5 to 6 Angstromsthick which contacts CoFeB layer 52, and an upper Ru layer 41 b. A Rucapping layer can cause a sizable enhancement of the damping constant inan adjacent CoFeB free layer as presented by Y. Tserkovnyak et al. in“Enhanced Gilbert damping in thin ferromagnetic films”, Phys. Rev.Lett., V 88, p 116601 (2002). Because of the relationship expressed inequation (2) mentioned previously, it is feasible to modify the freelayer/cap structure so as to reduce the damping parameter of the upperCoFeB layer 52 in free layer 40 and thereby reduce the intrinsiccritical current density (Jc) to a desirable value. We have discoveredthat reducing the NCC layer 51 thickness to 6 to 7 Angstroms andinserting a FeSiO layer 41 a about 5 to 6 Angstroms thick as the lowerlayer in capping layer 41, the damping parameter of the upper CoFeBlayer 52 is effectively lowered. Results will be described in a latersection with regard to Table 3.

Referring to FIG. 5, hard mask layer 42 may be a composite comprised ofa lower MnPt layer 42 a that contacts capping layer 41 and an upper Talayer 42 b on the MnPt layer. The MnPt/Ta configuration is designedespecially for RIE processes used to pattern the MTJ nanopillars of thisinvention. Ta layer 42 b thickness is from 300 to 500 Angstroms, andpreferably 300 Angstroms, while MnPt layer 42 a thickness is from 200 to300 Angstroms, and preferably 250 Angstroms. The MnPt layer 42 a isemployed to avoid using a 600 Angstrom thick Ta hard mask which wouldrequire a thicker photoresist layer (lower pattern resolution) duringthe hard mask patterning process. Thus, the photoresist pattern (notshown) is transferred through the Ta layer 42 b with a first RIE step.Thereafter, the photoresist layer is stripped and the pattern in the Talayer is transferred through the MnPt layer 42 a and underlying MTJlayers 33-41 with a second RIE step that has a substantially higher etchrate (relative to the Ta etch rate) than the first RIE step. The Taupper layer 42 b has sufficient thickness to prevent excessive thinningof the MTJ stack of layers during subsequent processing steps. It iswell known that variations in MTJ stack height because of excessivethinning during a chemical mechanical polish (CMP) step, for example,can degrade device performance.

The bottom electrode layer 32 and MTJ layers 33-42 may be sequentiallydisposed on a substrate that is an insulation layer 31 made of siliconoxide, alumina, or the like and comprising a via stud 30. It should beunderstood that the via stud 30 is connected to a transistor drain (notshown) in an underlying sub-structure. The transistor is typically usedin a write or read process that determines the resistance state of thebit cell (MTJ) once the MTJ stack of layers is patterned to form a MTJnanopillar structure and a bit line is formed on the MTJ nanopillar.Note that unlike conventional MRAM, magnetization switching in a STT-RAMMTJ is accomplished by passing current through a bit cell and not byfields induced by current in a word line and a bit line. The bottomelectrode 32 may have an area size in the “x, y” plane greater than thatof overlying MTJ layers 33-42.

The MTJ stack comprised of layers 33-42 may be formed in the sameprocess tool as the bottom electrode layer 32. For instance, the bottomelectrode 32 and MTJ stack may be formed in an Anelva C-7100 thin filmsputtering system or the like which typically includes three physicalvapor deposition (PVD) chambers each having five targets, an oxidationchamber, and a sputter etching chamber. At least one of the PVD chambersis capable of co-sputtering. Usually, the sputter deposition processinvolves an argon sputter gas and the targets are made of metal oralloys to be deposited on a substrate. The bottom electrode layer 32 andoverlying MTJ layers 33-42 may be formed after a single pump down of thesputter system to enhance throughput. The NOX process used to form theMgO barrier layer 39 is typically performed in an oxidation chamberwithin the sputter deposition tool.

Once the MTJ stack of layers 33-42 is laid down on the patterned BE 32,a high temperature annealing may be performed. For example, MTJ layers33-42 may be annealed in a vacuum by applying a magnetic field of 5000to 10000 Oe in magnitude along the x-axis (easy axis) for 1 to 5 hoursat a temperature of about 330° C. to 360° C.

Thereafter, an array of MTJ elements with essentially vertical sidewallsmay be fabricated by a process involving a RIE process as mentionedpreviously. First, a photoresist layer (not shown) is coated on the hardmask 42 and then patterned by a conventional photolithography process. Apattern of islands is formed in the photoresist layer wherein eachisland corresponds to the desired ellipse shape or another shape of theMTJ nanopillar from a top view. The patterned photoresist layer thenfunctions as an etch mask during a first reactive ion etch (RIE) processin a RIE system mainframe which removes uncovered regions of the Ta hardmask layer 42 b. The photoresist layer may be stripped and a second RIEprocess is employed to selectively etch through regions of MnPt hardmask layer 42 a, capping layer 41, and underlying MTJ layers 33-40 thatare not protected by hard mask 42. As a result, the pattern of islandsinitially formed in the photoresist layer is transferred through the MTJstack of layers to form an array of MTJ nanopillars. Since a RIE processis used to generate essentially vertical sidewalls in the MTJnanopillars, a more dense array of MTJs is possible than when an ionbeam etch (IBE) is employed as in the prior art.

Referring to FIG. 8, the present invention anticipates an additionalembodiment wherein the BE layer is not patterned before the MTJ stack oflayers is deposited. Instead, the second etch process described above isextended so that the etch continues through the BE layer and forms a BE32 having essentially the same shape from a top view as the overlyingMTJ nanopillar. This embodiment allows a greater MTJ nanopillar densityin the STT-RAM array than in the previous embodiment (FIG. 2). In thisembodiment, a via stud (not shown) preferably contacts the lower surfaceof BE 32.

In both embodiments, following formation of an array of MTJ nanopillars,a second insulation layer (not shown) may be deposited on the BE 32and/or substrate 31 to a level that fills in the spaces between adjacentMTJ nanopillars. A CMP process is used to remove an upper portion of thesecond insulation layer and thereby expose the hard mask layer 42. Thus,the second insulation layer becomes coplanar with the hard mask layer.Then a conductive material such as Cu, Al, or alloys thereof may bedeposited on the second insulation layer and over the hard mask 42.Next, a bit line 43 that contacts the hard mask 42 is delineated bypatterning the conductive layer using a well known photoresistpatterning and etching sequence.

To determine the performance of a MTJ based on a prior art free layerand AP1 pinned layer combined with a MgO tunnel barrier made accordingto a method of the present invention, an unpatterned stack of layersrepresented by BE/NiCr45/MnPt150/Co₇₅Fe₂₅23/Ru7.5/AP1/Mg8-NOX—Mg4/freelayer/cap was probed by CIPT and B—H looper and the resultingmeasurements are shown in Table 1. The MTJ stack for all configurationsshown in Table 1 is formed on a NiCr45/Ru200/Ta150 bottom electrode andhas a 100 Angstrom thick Ru capping layer which is used in this exampleonly for the purpose of establishing good electrical contact for a CIPTmeasurement. The MgO layer was formed by first depositing an 8 Angstromthick Mg layer followed by a NOX process (1 torr, 1 slm O₂ for 100seconds) and then deposition of a 4 Angstrom thick Mg layer. Annealingwas performed for 1 hour (10K Oe) at the indicated temperature.

TABLE 1 Magnetic Properties of MTJs with BE/NiCr45/MnPt150/Co₇₅Fe₂₅23/Ru7.5/AP1/Mg8(NOX)Mg4/free layer/cap configuration Anneal Row AP1 (1hour) Free layer RA MR Bs Hc He Hk 1 Co₆₀Fe₂₀B₂₀21 360 C. Co₆₀Fe₂₀B₂₀209 166 0.62 11.5 −10 40.3 2 Co₄₀Fe₄₀B₂₀21 360 C. Co₄₀Fe₄₀B₂₀20 10 1800.65 12.3 −9.2 39.5 3 Co₄₀Fe₄₀B₂₀21 265 C. Co₄₀Fe₄₀B₂₀20 10 91 0.49 9.66.5 33.6 4 Co₄₀Fe₄₀B₂₀15-CoFe₂₅6 265 C. Co₄₀Fe₄₀B₂₀20 9 103 0.55 11.03.7 34

Note that the free layer in row 1 is made of Co₆₀Fe₂₀B₂₀ whichcorresponds to the free layer in the Hosomi reference mentionedpreviously, and the Co₄₀Fe₄₀B₂₀ free layer in row 2 corresponds to thatin the Hayakawa reference mentioned previously. For the 360° C. annealedMTJs, MR or dR/R (intrinsic) is at least equivalent to the valuesreported in the Hosomi and Hayakawa references. In rows 3 and 4 that areannealed at 265° C., the CoFeB free layer is mostly in the amorphousstate which leads to a lower MR value than in rows 1 and 2. It should bepointed out that the AP1 in row 4 is comprised of amorphous Co₄₀Fe₄₀B₂₀and crystalline Co₇₅Fe₂₅ wherein the crystalline Co₇₅Fe₂₅ interfaceswith the MgO tunnel barrier to enhance dR/R compared with row 3.

In a related experiment, a MTJ nanopillar shown in row 4 where theCo₄₀Fe₄₀B₂₀ free layer is 18 Angstroms thick and the hard mask isMnPt300/Ta300 was fabricated in different sizes. Dimensions of theresulting devices referred to as D2 and D3 are 100 nm×150 nm (oval) and100 nm×200 nm (oval), respectively. As measured by quasistatic (i.e.τ_(a)˜30 ms) testing, Rp for the D2 nanomagnet is around 1000 ohms andVbd (breakdown voltage) is around 800-850 mV. Hc is about 100 Oe and Ho(offset field) is approximately 0 for the D2 nanomagnet. Note thatHc˜100 Oe in the 265° C. annealed nanomagnet is much higher than Hc<100Oe shown in the Hayakawa reference for 270° C. and 300° C. annealednanomagnets. Obviously, Hayakawa's Co₄₀Fe₄₀B₂₀ free layer is amorphousas claimed while the high Hc for the D2 device indicates the 265° C.annealed Co₄₀Fe₄₀B₂₀ free layer is at least partially recrystallized.This result was confirmed by HR-TEM which shows CoFeB is mostlyamorphous with partial crystalline character for an 18 Angstrom thickCo₄₀Fe₄₀B₂₀ free layer deposited on NOX—MgO by a method describedherein. It is believed that the MgO tunnel barrier formed by a NOXprocess on a Mg layer yields a crystalline and highly (001) orientedlayer that can act as a template for CoFeB recrystallization. In theexample of the 265° C. annealed CoFeB layer, it is most likely that thelower portion of the free layer contacting the MgO tunnel barrier is theCoFeB portion that has recrystallized.

The dR/R for the D2 nanomagnet is as high as 80% which is significantlyhigher than the 49% reported for Hayakawa's 270° C. annealed nanomagnet.This result tends to substantiate our claim that the CoFeB portion atthe MgO/CoFeB free layer interface has been recrystallized. It should beunderstood that the 80% MR value for the patterned D2 device is not ashigh as the same MTJ configuration in an unpatterned stack (row 4 inTable 1). We observed the critical switching voltage (Vc⁺ for AP to P,Vc⁻ for P to AP) for switching the nanomagnet with an 18 Angstrom thickCo₄₀Fe₄₀B₂₀ free layer to be about 0.7V. High speed (pulse width τ_(a)down to 100 μs, 100 ns, and 10 ns) measurements were made on thequasistatic “switchable” D2, D3 nanomagnets. At high speed (τ_(a)=10ns), Vc_ave measured for these nanomagnets is around 1.4 V while the Vbdis also increased to around 1.7 V. The value of the intrinsic switchingcurrent density Jc₀ ⁺, Jc₀ ⁻, according to equationJc(τ)=Jc₀[1−k_(B)T/K_(u)V In(τ/τ₀)] can be obtained by extrapolating theswitching current density Jc^(+,−) to a pulse width of τ₀=1 ns whereτ₀=1 ns is the inverse of the activation frequency. We found that Jc₀_(—) ave, (Jc₀ ⁺+Jc₀ ⁻/2), is 7.0×10⁶ A/cm² for the Co₄₀Fe₄₀B₂₀nanomagnet. Thermal stability factor K_(u)V/k_(B)T is around 42.However, Jc₀ of 7.0×10⁶ A/cm² is too high for STT-RAM devices.

As mentioned previously, we have also fabricated a DSF nanomagnet toreduce Jc₀. In particular, a configuration(A)=BE/NiCr45/MnPt150/CoFe23/Ru7.5/CoFeB15-CoFe6(AP1)/MgO—NOX/Co₄₀Fe₄₀B₂₀20/Cu30/CoFe20(pinned)/MnPt250/Ta(HM)and a configuration (B) which isBE/NiCr45/MnPt150/CoFe23/Ru7.5/CoFeB15-CoFe6(AP1)/MgO—NOX/Fe3-Co₄₀Fe₄₀B₂₀9-Fe6/MgO—NOX/CoFe20(pinned)/MnPt250/Ta(HM)were made and evaluated. Although Jc₀ for the (A) nanomagnet and (B)nanomagnet was lowered to 4.0×10⁶ A/cm² and 2.5×10⁶ A/cm², respectively,the dR/R was unfortunately reduced to 40% and 35%, respectively.

Table 2 shows magnetic properties of MTJs comprised of aCoFeB/FeSiO/CoFeB free layer configuration, MgO (NOX) tunnel barrier,and a Ru capping layer according to one embodiment of the presentinvention. In this case, a Co₄₀Fe₄₀B₂O/Co₇₅Fe₂₅ AP1 pinned layer isemployed because the CoFeB free layer in rows 1-7 remains amorphousafter 265° C. annealing. In the unpatterned stacks, a CIPT measurementis taken to determine MR and no hard mask is required for the test. Forrows 1 and 2, the MR values with an asterisk were measured on patternedMTJ nanomagnets having a MnPt/Ta configuration according to oneembodiment of the present invention. The MTJ layers and thicknesses arelisted in the header for Table 2. Annealing was performed with anapplied field of 10000 Oe with the time and temperature indicated.

TABLE 2 Magnetic Properties of MTJs with BE/NiCr45/MnPt150/Co₇₅Fe₂₅24.5/Ru7.5/CoFeB15—CoFe6/Mg8(NOX)Mg4/free layer/Ru30(cap) Row NOX Anneal Freelayer RA MR Bs Hc Hin Hk 1 1 torr, 1slm 265 C., 2 hrCoFeB14/FeSiO15/CoFeB6 15 68/55* 0.59 12.5 6.5 28.3 300″ 2 1 torr, 1slm265 C., 2 hr CoFeB14/FeSiO10/CoFeB6 12 78/63* 0.60 13.3 7.9 33.0 300″ 31 torr, 1slm 265 C., 2 hr CoFeB14/FeSiO8/CoFeB6 11 85 0.60 13.1 7.9 32.5300″ 4 1 torr, 1slm 265 C., 2 hr CoFeB14/FeSiO6/CoFeB6 10 101 0.60 13.36.5 35.1 300″ 5 1 torr, 1slm 265 C., 2 hr Fe3/CoFeB12/FeSiO10/Fe6 12 960.60 13.0 10.6 32.0 300″ 6 1 torr, 1slm 265 C., 2 hrCoFeB14/FeSiO10/CoFeB6 7 75 0.58 8.55 15.2 37.9 100″ 7 100 sccm, 265 C.,2 hr CoFeB14/FeSiO10/CoFeB6 6 77 0.59 9.31 16.9 38.8 600″ 8 1 torr, 1slm360 C., 1 hr CoFeB14/FeSiO10/CoFeB6 12 127 0.65 22.2 6.5 34.6 300″

Values with an asterisk (*) are those for patterned samples. All otherresults are for unpatterned samples. Our process of record (POR) NOXmethod is indicated by the NOX conditions listed in rows 1-5 where an 8Angstrom thick Mg film is exposed to 1 torr oxygen pressure using a 1slm O₂ flow rate for 300 seconds. Note that in rows 2-5, RA is more orless stabilized at about 11 ohm-μm² when the FeSiO thickness ismaintained in the range of 6 to 10 Angstroms. As stated earlier withregard to the NCC layer 51 in FIG. 3, NCC thickness is preferably keptat or below the minimum Fe(Si) granule size which has been determined tobe approximately 10 Angstroms. As indicated in row 1 of Table 2, RAincreases and dR/R decreases for a thicker (15 Angstroms) FeSiO layerwhich means some of the Fe(Si) granules are not functioning asnanocurrent channels.

Nanomagnets were constructed from row 1 and row 2 MTJ configurations inTable 2. A hard mask layer comprised of MnPt300/Ta500 was formed on theRu30 cap layer to facilitate the RIE process to pattern the MTJ stacks.For the D2 devices (100 nm×150 nm ovals), quasistatic testing showsHc˜100+/−25 Oe, Ho˜0 Oe, Vbd˜825+/−50 mV, and Vsw-ave˜300 mV. Rp wasmeasured for the D2 devices and is 1000 ohm and 1500 ohm, respectively,for the FeSiO10 MTJ and FeSiO15 MTJ. Rp_cov was around 4% for both ofthe D2 devices. As shown in the MR column of Table 2, dR/R of thenanomagnets is 63% and 55% for row 2 and row 1, respectively. High speedmeasurement down to 10 ns yielded Vc_ave˜600 mV and Vbd increasedto >1.6 V, thereby showing good write margin. Jc₀ for the row 2nanomagnet was determined to be 2.5×10⁶ A/cm², which is a 3× reductioncompared with a single MTJ fabricated with a single CoFeB free layer(7.0×10⁶ A/cm²) in the prior art.

Using a 4 Kb STT-RAM circuit design, nanomagnets fabricated with aCoFeB14-FeSiO15-CoFeB6 free layer (MTJ 1) and a CoFeB14-FeSiO10-CoFeB6free layer (MTJ 2) were studied. While MTJ (1) displayed a wide Rpdistribution as illustrated in FIG. 7 a, MTJ (2) is characterized ashaving a significantly narrower Rp distribution as depicted in FIG. 7 b.In both FIG. 7 a and FIG. 7 b, delta R/Rp is uncorrected and has a sigmaof 2%. Note that a dR/Rp of 0.49 means a dR/Rp of 49%. By correcting fortransistor resistance which is about 170 ohm, the corrected dR/Rp forMTJ (2) should be 49%×(1000 ohm/1000 ohm−170 ohm)=59%. As mentionedpreviously, when FeSiO layers are formed thicker than 10 Angstroms whichis the minimum size of Fe(Si) granules, some of the Fe(Si) granules failto function as nano-conducting channels between upper and lower CoFeBlayers thereby causing R to increase compared with a FeSiO layer in the6 to 10 Angstrom thickness range.

Referring to FIG. 6, anti-parallel current (lap) is plotted vs.frequency for 4 Kb devices. Low R from MTJs having FeSiO layers in the6-10 Angstrom thickness range correspond to high lap values shown inrectangle 61 in the lower right plot. Data points in rectangle 61translate to points within area 63 in the upper left graph wherebreakdown voltage (Vbd) is plotted vs. switching voltage (Vc). We findthat devices with low R (high lap) have tighter distribution and goodwrite margin. It is desirable for a data point to be substantially belowthe diagonal line in the upper left plot so that switching voltage isconsiderably less than breakdown voltage. Devices with high R (low lap)represented by data points within the larger rectangle 60 in the lowerright graph have a wider distribution and poor write margin. Pointswithin rectangle 60 translate to data points within area 62 in the upperleft graph.

The conclusion derived from FIG. 6 and FIGS. 7 a, 7 b is that betterwrite margin (i.e. well separation between Vc and Vbd) can be achievedin lower RA MTJs. Since Vc is proportional to Rp, it follows that Vc isreduced with lower RA MTJs. As shown in MTJs represented by rows 6 and 7in Table II, we have discovered that using lower oxygen pressure orreducing oxidation time in the NOX process leads to a considerablereduction in RA. It is also important to note that lowering RA does notsacrifice dR/R. Using a 4 Kb STT-RAM circuit design, nanomagnets made ofrow 7 configuration with an RA of about 6 ohm-μm² were fabricated. Weobserved that Vc_ave was greatly reduced to ˜400 mV while Vbd was notaffected. Thus, we have demonstrated a good write margin in low RAdevices.

The best Rp_cov in or STT-RAM devices is around 4% which is equivalentto that of the Hosomi reference in the prior art. For this STT-RAM, readmargin (MR/Rp_cov) is 60%/4%=15, too low to be useful for STT-RAMproduct applications. To have a read margin>20 as desired, dR/R>100% isrequired. As indicated in row 8 of Table 2, dR/R (intrinsic)=127% isachieved for a MTJ comprised of a CoFeB/FeSiO/CoFeB free layer that isannealed using conditions comprised of 360° C. and 10K Oe annealing for1 hour. This MR result would ensure an acceptable read margin>20 forproduct applications. Although RA increased to 12 ohm-μm² for thisdevice, we have indicated earlier that the NOX process conditions may bemodified to lower RA to 6 or 7 ohm-μm² without compromising dR/R.

According to a second embodiment of the present invention, a FeSiO layermay be inserted between the upper CoFeB free layer and the Ru cap layerto offset the tendency of Ru to enhance the damping constant of theadjacent CoFeB free layer. Table 3 shows the magnetic performanceproperties of a MTJ fabricated with a Co₄₀Fe₄₀B₂₀14/FeSiO6/Co₄₀Fe₄₀B₂₀6(NCC) free layer that is capped by FeSiO6/Ru30. The FeSiO layer in thefree layer is preferably thinned to about 6 to 7 Angstroms when a 5-6Angstrom thick FeSiO layer is employed in the capping layer in order tominimize total FeSiO thickness in the MTJ stack and prevent R fromincreasing. Comparing row 2 to row 1, a high MR is maintained with asimilar RA by annealing the MTJ with two FeSiO layers at 330° C. (10KOe) for 1 hour.

TABLE 3 Magnetic Properties of MTJs with BE/NiCr45/MnPt150/Co₇₅Fe₂₅24.5/Ru7.5/CoFeB15—CoFe6/Mg8(NOX)Mg4/free layer/cap configuration Anneal RowFree layer Cap layer (1 hour) RA MR Bs Hc He Hk 1 CoFeB14/FeSiO10/CoFeB6Ru30 360 C. 12 127 0.65 22.2 6.5 34.6 2 CoFeB14/FeSiO6/CoFeB6 FeSiO6/330 C. 11 127 0.66 16.3 −0.4 22.6 Ru30

According to the present invention, the combination of a thin Ru orFeSiO/Ru capping layer, a MgO (NOX) tunnel barrier, and a composite freelayer which includes a NCC layer sandwiched between two CoFeB layerswhere the top CoFeB layer is thinner than the bottom CoFeB layer resultsin a high dR/R with low critical current density Jc₀ as well as goodwrite margin and read margin>20 that is necessary for STT-RAM productapplications. This performance is an improvement over the prior artwhere not all of the aforementioned properties have been achieved at thesame time. The CoFeB/FeSiO/CoFeB free layer and FeSiO/Ru capping layercan be readily implemented with existing tools and processes. A methodhas been provided such that the MTJ nanopillar design described hereincan be easily reproduced.

While this invention has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this invention.

We claim:
 1. A Magnetic Tunneling Junction (MTJ) nanopillar structureformed on a bottom electrode in a Spin Torque Transfer Random AccessMemory (STT-RAM) device, comprising: (a) a seed layer formed on thebottom electrode; (b) an anti-ferromagnetic (AFM) layer formed on theseed layer; (c) a synthetic anti-ferromagnetic (SyAF) pinned layer witha second antiparallel (AP2)/coupling/first antiparallel (AP1)configuration formed on the AFM layer wherein the AP2 layer contacts theAFM layer; (d) a MgO tunnel barrier layer contacting the AP1 layer; (e)a composite free layer having a lower magnetic layer, an upper magneticlayer, and a middle nanocurrent channel (NCC) layer with magneticchannels connecting the upper and lower magnetic layers wherein NCC is ananocurrent channel layer comprised of R(Si) grains having a minimumdiameter that are formed in an oxide or nitride insulator matrix and Ris Fe, Ni, Co, a metal M or a combination thereof, said lower magneticlayer contacting the MgO layer has a greater thickness than that of theupper magnetic layer and the NCC layer has a thickness which is lessthan or equal to the R(Si) grain minimum diameter; and (f) a cappinglayer disposed on the composite free layer.
 2. The MTJ nanopillarstructure of claim 1 wherein the bottom electrode is comprised of Ta ora composite layer with an uppermost Ta layer, and an oxygen surfactantlayer (OSL) is disposed on the Ta layer, said seed layer contacts anupper surface of said OSL.
 3. The MTJ nanopillar structure of claim 1wherein the insulator matrix is SiO₂, the R(Si) grain minimum diameteris about 10 Angstroms, and the thickness of the NCC layer in thecomposite free layer is about 6 to 10 Angstroms.
 4. The MTJ nanopillarstructure of claim 1 wherein the lower magnetic layer in the compositefree layer is CoFeB, and the upper magnetic layer is CoFeB or CoFe. 5.The MTJ nanopillar structure of claim 1 wherein the upper magnetic layerin the composite free layer has a thickness from about 6 to 8 Angstromsand the lower magnetic layer in the composite free layer has a thicknessbetween about 10 and 15 Angstroms.
 6. The MTJ nanopillar structure ofclaim 1 wherein the capping layer is comprised of a NCC layer thatcontacts the upper magnetic layer in the composite free layer, said NCClayer has a thickness from about 5 to 6 Angstroms.
 7. The MTJ nanopillarstructure of claim 1 wherein the capping layer is comprised of Ru, and ahard mask having a MnPt/Ta or Ta configuration contacts the cappinglayer.
 8. The MTJ nanopillar structure of claim 7 wherein the MnPt layerhas a thickness from about 200 to 300 Angstroms.
 9. The MTJ nanopillarstructure of claim 1 wherein the AP1 layer is CoFeB, CoFe, or acombination thereof.
 10. The MTJ nanopillar structure of claim 4 whereinthe lower magnetic layer in the composite free layer is CoFeB with acomposition represented by Co₄₀Fe₄₀B₂₀.